Aircraft structures must be capable of reacting loads in a variety of different directions. For example, an aircraft wing must be capable of reacting aerodynamic loads that are imposed on the wing during flight. Such aerodynamic loads include bending loads that are reacted by the wing skin and the internal wing structure. Bending loads are typically reacted by stiffeners or stringers in the internal wing structure. Stringers are coupled to the wing skin and generally extend in a span-wise direction along the wing interior. Stringers may be provided in a variety of different cross-sectional shapes and sizes including, but not limited to, an I-beam shaped cross section and/or a hat-shaped cross-section. A hat-shaped stringer includes a pair of webs which extend upwardly from a base portion of the stringer and are interconnected by a cap to enclose the hat-shaped cross-section.
The internal structure of a wing typically further includes a series of ribs which maintain the aerodynamic shape of the wings and/or assist in distributing loads that are imposed on the wings. Ideally, the wing ribs are interconnected to the stringers in order to increase the load carrying capability of the wing. One of the more structurally efficient arrangements for interconnecting the ribs to the stringers is by directly connecting the ribs to the hat-shaped stringers such as by mechanical attachment.
Although conventional metallic ribs may be directly connected to conventional metallic stringers at the stringer cap, hat stringers formed of composite materials may lack sufficient load-carrying capability to connect the rib to the cap due to the orientation of the loads relative to the length of the stringer. More specifically, pulloff loads are oriented in an out-of-plane direction or perpendicularly relative to the length of the stringer. Pulloff loads may occur as a result of operational loads and/or maneuver loads imposed on the wing structure. Operational loads may include overpressure loads resulting from the mass of the fuel in the wing tanks when the tanks are fully filled. Maneuver loads may include inertial loads resulting from movement of the fuel in the fuel tanks and from inertia acting on the mass of the fuel during certain aircraft maneuvers.
Because of an undesirably low pulloff capability of conventional wing stringers of composite construction, direct connection of the ribs to the stringer caps is avoided and the wing ribs are instead extended over and around the stringer cap and are directly mounted to the wing skin at the base of the stringer using shear ties. Although effective for their intended purposes, shear ties add significant weight to the aircraft due to the need for a shear tie at each location where a rib intersects with a stringer. The added weight of the shear ties at each rib-stringer intersection reduces the payload capacity of the aircraft and increases fuel consumption. In addition, the addition of a shear tie at each rib-stringer intersection increases manufacturing complexity, cost and production time.
As can be seen, there exists a need in the art for a system and method for directly connecting a rib to the cap of a stringer in order to obviate the need for an additional shear tie at each rib-stringer intersection. In this regard, there exists a need in the art for a system and method for connection of the ribs to the stringer cap in a manner that may sustain out-of-plane (i.e., pulloff) loads in an efficient manner. Ideally, the system for connecting the rib to the stringer cap is preferably low in cost, simple in construction and light in weight.